MR imaging has been established as a viable and unique imaging technique for cardiological examinations. Its utility is related to the ability to provide both precise morphology and assessment of global and regional ventricular function. For functional assessment of the heart, steady-state free-precession (SSFP) has become an indispensable sequence due to its superb blood-to-myocardium contrast (see e.g. Oppelt A, et al. FISP: a new fast MRI sequence. Electromedica 1986; 54: 15-18.). Compared to spoiled steady-state sequences, SSFP also offers improved signal-to-noise ratio (SNR), and it allows for a shorter minimum repetition time (TR). However, the advantages of SSFP come at the expense of increased susceptibility to experimental non-idealities. As a result, it is only with recent advances in scanner hardware and sequence design that SSFP has become widely available. One prominent artifact in SSFP imaging is the so-called banding artifact. This results from unaccounted dephasing of the magnetization, such as from inhomogeneity of the main magnetic field. To reduce susceptibility to banding artifacts, it is critical to keep TR as short as possible, so that there is less time for unaccounted dephasing to occur. This, in turn, limits the use of echo-planar readout and, accordingly, acquisition efficiency of SSFP imaging is lower than in multiplanar image formation (see Mansfield P. Multiplanar image formation using NMR spin echoes. J Phys C 1977; 10: L55-58.).
SSFP can be applied in a real-time mode or in a triggered mode. In real-time mode, data are acquired in a continuous fashion. The lower acquisition efficiency of SSFP limits the achievable spatial and temporal resolutions. In the triggered mode, the idea is to “freeze” the effects of anatomical motion, which would reduce image quality. This is achieved using cardiac triggering—the coupling of image acquisition to a trigger signal—for cardiac studies, generally to an ECG signal. Using the assumption that most biological motion is rhythmic, triggering permits data acquired at different cardiac cycles but at the same point within the cycle to be combined to improve signal-to-noise ratio in the final image or to aggregate sufficient data to reconstruct a fully-sampled image. Since the images from multiple cardiac cycles are acquired at the same point within the cycle, the effects of motion are minimized. However, residual motion does exist, due to cycle-to-cycle variation, and it is manifested as image artifacts. Therefore, it is desirable to reduce the overall scan duration in order to minimize such variability. A series of images acquired at different points of the cardiac cycle can also be played back in cinematic loop fashion, called cine imaging, to provide physicians a way of viewing dynamic activities. Triggered acquisitions are generally performed in conjunction with a fast imaging pulse sequence for improved temporal resolution. The lower acquisition efficiency of SSFP mentioned previously leads to a longer overall scan duration, compared to echo-planar imaging sequences, for example.
An important application of SSFP imaging is in obtaining volumetric time-resolved images of the heart. For example, multi-slice, multi-breathhold imaging based on SSFP is currently the method of choice for obtaining cine views of the heart. From these images, functional parameters can be determined with high accuracy and reproducibility (see e.g. W. Li et. al. MR assessment of left ventricular function: quantitative comparison of fast imaging employing steady-state acquisition (FIESTA) with fast gradient echo cine technique. J. Magn. Reson. Imaging, 2002; 16, p. 559-564). There are, however, limitations associated with multi-slice, multi-breathhold acquisitions. Inconsistencies in the multiple breath-holds may lead to misregistration of slices. Furthermore, the low bandwidth-time product of the excitation pulses used for short TR SSFP imaging results in large variations of excitation angles within the slice. This not only affects the signal-to-noise and contrast-to-noise ratios, it also hampers the reconstruction of multi-planar reformats through the heart due to the slab boundary artifact.
To overcome the aforementioned shortcomings, SSFP with true 3D imaging was previously developed (see B. A. Jung et. al. Single-breathhold 3D-true FISP cine cardiac imaging. Magn. Reson. Med. 2002; 48, p. 921-925). Cine 3D SSFP of the heart was shown to offer superior signal-to-noise and contrast-to-noise ratios compared to multislice sequences. However, in that work, compromises between image quality, spatial and temporal resolutions had to be made to accommodate sufficient volume coverage into a single breath-hold. Using an elliptical shutter in ky-kz space, cine volumetric data sets may be obtained at relatively high spatial resolution in a single breathhold. Still, temporal sampling does not satisfy the requirements for accurate left ventricular volume calculations. Generally, at least 10 temporal frequency harmonics are considered sufficient for accurate volume curve calculation, requiring a sampling rate of at least 20×HR/60 Hz, with HR being the actual heart rate.